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High Energy Physics - Phenomenology

Title:Lattice QCD input for axion cosmology

Abstract: One intriguing BSM particle is the QCD axion, which could simultaneously
provide a solution to the Strong CP problem and account for some, if not all,
of the dark matter density in the universe. This particle is a pNGB of the
conjectured Peccei-Quinn (PQ) symmetry of the Standard Model. Its mass and
interactions are suppressed by a heavy symmetry breaking scale, $f_a$, whose
value is roughly greater than $10^{9}$ GeV (or, conversely, the axion mass,
$m_a$, is roughly less than $10^4\ μ\text{eV}$). The density of axions in
the universe, which cannot exceed the relic dark matter density and is a
quantity of great interest in axion experiments like ADMX, is a result of the
early-universe interplay between cosmological evolution and the axion mass as a
function of temperature. The latter quantity is proportional to the second
derivative of the QCD free energy with respect to the CP-violating phase,
$θ$. However, this quantity is generically non-perturbative and previous
calculations have only employed instanton models at the high temperatures of
interest (roughly 1 GeV). In this and future works, we aim to calculate the
temperature-dependent axion mass at small $θ$ from first-principle lattice
calculations, with controlled statistical and systematic errors. Once
calculated, this temperature-dependent axion mass is input for the classical
evolution equations of the axion density of the universe. Due to a variety of
lattice systematic effects at the very high temperatures required, we perform a
calculation of the leading small-$θ$ cumulant of the theta vacua on large
volume lattices for SU(3) Yang-Mills with high statistics as a first proof of
concept, before attempting a full QCD calculation in the future. From these
pure glue results, the misalignment mechanism yields the axion mass bound $m_a
\geq (14.6\pm0.1) \ μ\text{eV}$ when PQ-breaking occurs after inflation.